The heart of a computer is a non-volatile magnetic storage device that is referred to as a hard disk drive. The hard disk drive includes a hard disk and an assembly of write and read heads. The assembly of write and read heads is supported by a slider that is suspended by a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the assembly of write and read heads over selected circular data tracks on the surface of the hard disk. The suspension arm biases the slider toward the surface of the hard disk, and an air bearing generated by the rotation of the hard disk causes the slider to fly on the air bearing at a very low elevation (fly height) over the surface of the hard disk. When the slider rides on the air bearing, the write and read heads write data to and read data from, respectively, the circular data tracks on the surface of the hard disk. Processing circuitry connected to the write and read heads then operates according to a computer program to implement writing and reading functions.
In a reading process, the read head passes over magnetic transitions of a data track on the surface of the hard disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of a read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor, and are then converted into voltage changes that generate read signals. The resulting read signals are used to decode data encoded in the magnetic transitions of the data track.
In a typical read head, a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR) read sensor is electrically separated by side oxide layers from longitudinal bias layers in two side regions to prevent a sense current from shunting into the two side regions, but is electrically connected with lower and upper shields to allow the sense current to flow in a direction perpendicular to the sensor plane. A typical CPP TMR read sensor comprises an electrically insulating barrier layer sandwiched between lower and upper sensor stacks. The barrier layer is formed by a nonmagnetic oxygen-doped Mg (Mg—O) or Mg oxide (MgOX). When the sense current quantum-jumps across the Mg—O or MgOX barrier layer, a TMR effect causes a change in the resistance of the CPP TMR read sensor. The strength of this TMR effect is typically characterized by a ratio of a TMR coefficient (ΔRT/RJ) divided by a product of junction resistance and area (RJAJ), where ΔRT is a maximal resistance change caused by the TMR effect, RJ is a junction resistance, and AJ is a junction area. A typical CPP GMR read sensor comprises an electrically conducting spacer layer sandwiched between the lower and upper sensor stacks. The spacer layer is formed by a nonmagnetic Cu or oxygen-doped Cu (Cu—O) film. When the sense current flows across the Cu or Cu—O spacer layer, a GMR effect causes a change in the resistance of the CPP GMR read sensor. The strength of this GMR effect is typically characterized by a GMR coefficient (ΔRG/RMin), where ΔRG is a maximal resistance change caused by the GMR effect, and RMin is a minimal resistance of the CPP GMR read sensor.
A typical lower sensor stack can comprise a first seed layer formed of a Ta film, a second seed layer formed of a nonmagnetic Ru film, a pinning layer formed of a antiferromagnetic Ir—Mn film a keeper layer formed by a ferromagnetic Co—Fe film, an antiparallel coupling layer formed of a Ru film, and a reference layer formed of a ferromagnetic Co—Fe—B film. The keeper layer, the antiparallel-coupling layer, and the reference layer form a flux-closure structure where four fields are induced. First, a unidirectional anisotropy field (HUA) is induced by exchange coupling between the antiferromagnetic pinning layer and the keeper layer. Second, an antiparallel-coupling field (HAPC) is induced by antiparallel coupling between the keeper layer and the reference layer and across the antiparallel-coupling layer. Third, a demagnetizing field (HD) is induced by the net magnetization of the keeper layer and the reference layer. Fourth, a ferromagnetic-coupling field (HF) is induced by ferromagnetic coupling between the reference layer and the sense layers and across the barrier or spacer layer. To ensure proper sensor operation, HUA and HAPC must be high enough to rigidly pin magnetizations of the keeper layer and the reference layer in opposite transverse directions perpendicular to an air bearing surface (ABS), while HD and HF must be small and balance with each other to orient the magnetization of the sense layers in a longitudinal direction parallel to the ABS.
An upper sensor stack can comprise a first sense layer formed of a ferromagnetic Co—Fe film, a second sense layer formed of a ferromagnetic Co—Fe—B film, and a cap layer such as a Ru film. The total magnetic moment of the Co—Fe first sense layer and the Co—Fe—B second sense layer is equivalent to the magnetic moment of a 4.5 nm thick ferromagnetic Ni—Fe film without moment losses at interfaces. The Co—Fe first sense layer acts as a buffer layer to prevent the Co—Fe—B second sense layer from B segregations at an interface between the barrier layer and the sense layers, thus facilitating the Co—Fe—B second sense layer to exhibit a strong TMR effect after annealing. The Co—Fe—B second sense layer exhibits an interstitial-type amorphous structure after its deposition, which transfers into a polycrystalline structure after annealing, thereby exhibiting the strong TMR effect. To attain the interstitial-type amorphous structure, its B content must be high enough for B atoms, which are much smaller than Co and Fe atoms, to occupy interstitial sites of a crystalline structure and thus interfere with the ability of the Co and Fe atoms to crystallize.
It should be noted that the sense layers do not comprise a ferromagnetic Ni—Fe (permalloy) film at all, which has conventionally formed at least part of sense layers of anisotropic and giant magnetoresistance sensors for more than thirty years due to its very soft ferromagnetic properties, such as a low easy-axis coercivity (HCE), a low hard-axis coercivity (HCH), a low anisotropy field (HK), as well as its negative saturation magnetostriction (λS). The use of Ni—Fe a sense layer has been prohibited since its direct contact with the Co—Fe or Co—Fe—B sense layer causes diffusion and substantially deteriorates the TMR effect. In spite of the fact that the Co—Fe first sense layer and the Co—Fe—B second sense layer exhibit non-satisfactory ferromagnetic properties and positive λS, they still facilitate the CPP TMR read sensor to exhibit a TMR effect.